1. Field of the Invention
The present invention relates to a driver circuit for driving a semiconductor light-emitting device for use in a light scanning recording apparatus, an optical communication system, or the like, and more particularly to a driver circuit for controlling a semiconductor light-emitting device to produce varying optical outputs with increased response.
2. Description of the Prior Art
There have widely been put to use light scanning recording apparatuses, which include light deflectors for deflecting a light beam to scan a photosensitive recording medium to record information thereon. Optical communication systems for transmitting an optical signal through an optical fiber have also been employed in practice. A semiconductor light-emitting device such as a semiconductor laser or a light-emitting diode, for example, has heretofore been used as one of the light sources for emitting light beams in such light scanning recording apparatuses and optical communication systems. The semiconductor light-emitting device has numerous advantages in that it is small in size, inexpensive, has a low power requirement, and can directly modulate an output laser beam by varying a drive current supplied thereto.
The semiconductor light-emitting device is energized by a driver circuit which is normally combined with an automatic power control (APC) circuit to control the light-emitting circuit to produce the exact quantity of light indicated by a light-quantity command signal for light output stability control. The conventional APC circuit detects either the quantity of light emitted backwards from the semiconductor light-emitting device (i.e., light emitted in a direction opposite to the direction of the light beam emitted for recording an image or the like, hereinafter referred to as the forwardly emitted light beam), or the quantity of light branched off from the forwardly emitted light beam by a half-silvered mirror or the like, and controls a drive current, which is supplied to the semiconductor light-emitting device, so as to eliminate any difference between the detected quantity of light and a targent quantity of light represented by the light-quantity command signal.
The quantity of light emitted backwards from the semiconductor light-emitting device or the quantity of light branched from the forwardly emitted light beam is considerably smaller than the total quantity of light emitted by the semiconductor light-emitting device. Since the level of a feedback signal used for the light output stability control tends to be low, it is necessary to greatly amplify the feedback signal. This requires a number of amplifying stages to be employed in the amplifier, and causes a large phase delay in the light-quantity control system. For this reason, it has proven difficult to employ a wide frequency band in the feedback control loop in the light-quantity control system, resulting in a poor control response.
The poor control response is problematic in that if the level of the light-quantity command signal varies stepwise up or down, then a certain time lag is experienced before the quantity of light emitted from the semiconductor light-emitting device reaches the new level of the light-quantity command signal, and this time lag presents a major obstacle to high-speed operation of the light scanning recording apparatus, the optical communication system, or the like in which the semiconductor light-emitting device is incorporated.